/in/foo.cc: In function 'void pr()':
/in/foo.cc:46:19: warning: iteration 7903 invokes undefined behavior [-Waggressive-loop-optimizations]
         if( !vis[i] ){
              ~~~~~^
/in/foo.cc:45:24: note: within this loop
     for( int i = 2 ; i <= M ; i ++ ){
                        ^